The prior art of driving brushless d.c. motors has for the most part used some kind of position signal which in turn is decoded to provide for the proper switching of the power delivered to the motor windings. Most cases use magnetic sensors (Hall Sensors) or optical sensors to provide rotor position sensing. There is one case which the applicant is aware of which processes or integrates the back-emf signal to produce a position signal. There have also been three phase brushless d.c. motor drive circuits which use the ratio of the back-emf signals of two of its phases to provide for the proper energizing of the third phase.
The attempt to provide for back-emf commutation of brushless d.c. motors has been active for a variety of reasons among them being the elimination of positioning sensors in the motor with their troublesome implementation and calibration. Also back-emf commutation can provide for greater realiability and efficiency over speed and torque ranges. In the past back-emf commutation, has been too complicated and costly for widespread use. A disadvantage of back-emf commutation is the necessity of a separate starting circuit to provide torque to the motor until a sufficient back-emf is generated to operate self-commutation. There exist although methods which will provide a starting mode for self-commutating motors. For example the motor may be run as a stepper motor for an initial period.